In humans, all vital organs are protected within a body wall formed by ribs, vertebrae, and layers of thick muscle. Within the confined spaces vital organs such as the heart and lung, however, have to move and change volume continuously to function. For the best protection and function of the lung, the thoracic cage is shaped like a bellows with the diaphragm as the moving part. Moreover, the outer surface of the lung and the inner surface of the thoracic cage are covered by an elastic serous membrane with a smooth and lubricating surface to form a pleural cavity. This is almost like inserting a sealed wet and stretchable plastic bag between the lung and the thoracic wall to decrease friction. Thus, the pleura and pleural cavity are essential for the efficient function of the lung, as are the pericardium and pericardial cavity for the heart. This arrangement is so important that in the embryo, a body cavity lined by a serous membrane is formed, and the measures needed to maintain the cavity open are implemented before the vital organs develop.